Wall covering materials used in homes, hotels or office buildings mainly fulfil a decorating purpose. Wall covering materials are made, for instance, of fabrics or papers with a PVC coating on one side. The vinyl coating is printed or embossed in order to create different designs and textures. Because of the thinness and high density of a wall covering material it does not contribute to the thermal insulation feature provided by the walls or ceilings the wall covering material is attached to. On the other side, the wall coverings plays an important rule in thermal interactions between the walls and the room's interior. The walls release radiant heat into the room and receive radiant heat from the room's interior parts. The wall covering is the part of the wall which is mainly involved in this thermal interaction. The radiant heat absorbed by the wall covering is transported into the walls and the ceiling by conduction. Furthermore, the heat flux from the floor to the ceiling interacts with the wall covering system in form of convective heat. As a result, being in the forefront regarding the heat exchange between the wall and the ceiling and the room interior the wall covering should be considered for providing a thermo-regulating feature. A thermo-regulating feature supplied by the wall coverings leads to an substantial improvement of the thermal comfort of rooms. Furthermore, a thermo-regulating effect provided by the wall covering system results in a decrease in air-conditioning and heating demands which leads to substantial energy savings as well. A thermo-regulating effect can be provided by the application of phase change material.
Phase change material is a highly-productive thermal storage medium which possesses the ability to change its physical state within a certain temperature range. When the melting temperature is obtained during a heating process, the phase change from the solid to the liquid state occurs. During this melting process, the phase change material absorbs and stores a large amount of latent heat. The temperature of the phase change material remains nearly constant during the entire process. In a cooling process of the phase change material, the stored heat is released into the environment in a certain temperature range, and a reverse phase change from the liquid to the solid state takes place. During this crystallization process, the temperature of the phase change material also remains constant. The high heat transfer during the melting process and the crystallization process, both without any temperature change, is responsible for the phase change material's appeal as a source of heat storage.
In order to contrast the amount of latent heat absorbed by a phase change material during the actual phase change with the amount of sensible heat in an ordinary heating process, the ice-water phase change process will be used. When ice melts, it absorbs an amount of latent heat of about 335 J/g. When the water is further heated, it absorbs a sensible heat of only 4 J/g while its temperature rises by one degree C. Therefore, the latent heat absorption during the phase change from ice into water is nearly 100 times higher than the sensible heat absorption during the heating process of water outside the phase change temperature range.
In addition to ice (water), more than 500 natural and synthetic phase change materials are known. These materials differ from one another in their phase change temperature ranges and their latent heat storage capacities.
Currently, only crystalline alkyl hydrocarbon phase change materials having different chain lengths are used for finishing yarns, textiles and foams. Characteristics of these phase change materials are summarized in Table 1.
TABLE 1Crystalline alkyl hydrocarbonsLatent heatCrystallineMeltingCrystallizationstoragealkyltemperature,temperature,capacity,hydrocarbonsFormula° C.° C.J/gEicosaneC20H4236.130.6247NonadecaneC19H4032.126.4222OctadecaneC18H3828.225.4244HeptadecaneC17H3621.716.5213
The crystalline alkyl hydrocarbons are either used in technical grades with a purity of approximately 95% or they are blended with one another in order to cover specific phase change temperature ranges. The crystalline alkyl hydrocarbons are nontoxic, non-corrosive, and non-hygroscopic. The thermal behavior of these phase change materials remains stable under permanent use. Crystalline alkyl hydrocarbons are byproducts of petroleum refining and, therefore, inexpensive.
Salt hydrates are alloys of inorganic salts and water. The most attractive properties of salt hydrates are the comparatively high latent heat values, the high thermal conductivities and the small volume change during melting. Salt hydrates often show an incongruent melting behaviour which results in a lack in reversible melting and freezing making them unsuitable for permanent use. Salt hydrates with reversible melting and freezing characteristics are summarized in Table 2.
TABLE 2Salt hydratesMeltingLatent heat storagetemperature,capacity,Salt hydrates° C.J/gCalcium Cloride Hexahydrate29.4170Lithium Nitrate Trihydrate29.9236Sodium Sulfate Decahydrate32.4253
In the present applications of the phase change material technology in textiles, only crystalline alkyl hydrocarbon are used which are microencapsulated, i.e., contained in small micro-spheres with diameters between 1 micron and 30 microns. These microcapsules with enclosed phase change material are applied to a textile matrix by incorporating them into acrylic fibers and polyurethane foams or by coating them onto textile surfaces.
U.S. Pat. No. 4,756,958 reports a fiber with integral micro-spheres filled with phase change material which has enhanced thermal properties at predetermined temperatures.
U.S. Pat. No. 5,366,801 describes a coating where micro-spheres filled with phase change material are incorporated into a coating compound which is then topically applied to fabric in order to enhance the thermal characteristics thereof.
U.S. Pat. No. 5,637,389 reports an insulating foam with improved thermal performance, wherein micro-spheres filled with phase change material are embedded.
The micro-encapsulation process of crystalline alkyl hydrocarbon phase change materials is a very time-consuming and complicated chemical process running over several stages making the microcapsules with enclosed phase change material very expensive.
There are several thermal effects which can be obtained by a phase change material application in a certain product, such as:                A cooling effect, caused by heat absorption of the phase change material.        A heating effect, caused by heat emission of the phase change material.        A thermo-regulating effect, resulting from either heat absorption or heat emission of the phase change material.        
The efficiency of each of these effects is determined by the latent heat storage capacity of the phase change material, the phase change temperature range and the structure of the carrier system.
The total latent heat storage capacity of the phase change material in a certain product depends on the phase change material's specific latent heat storage capacity and its quantity. In order to obtain a successful phase change material application, the phase change temperature range and the application temperature range need to correspond.
In addition, performance tests carried out on textiles with phase change material have shown that the textile substrate construction also influences the efficiency of the thermal effects obtained by the phase change material. For instance, thinner textiles with higher densities readily support the cooling process.